This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 00 124 253.6 filed in Europe on Nov. 10, 2000; the entire content of which is hereby incorporated by reference.
The present invention relates to processes for the reduction of artefacts in the reproduction or display of images, especially photographic images which include an multitude of image points. The invention further relates to image reproducing devices, such as printers (inkjet printers, laser printers, thermal printers, and so on), monitors (for example, ICD monitors or cathel gray monitors or CRT monitors) and devices for the capturing of images on radiation sensitive (especially light sensitive) media (for example photographic paper), for example photoprinters or DMD""s (digital mirror devices) functioning with lasers or monitors. Particularly, the invention relates to image reproducing devices which reproduce photographs, whereby the reproduction of photographs also includes the generation of photographs on media, especially photographic paper or normal paper. The image data are especially photographic image data, which were captured with an image capturing apparatus, such as, for example, a photographic apparatus, a camera (for example film camera, digital camera, video camera).
The invention further relates to photographic labs, especially minilabs, which use an image reproducing device, an error correction process or a large space lab.
The invention relates further to programs which carry out error correction process.
A high quality is demanded especially in the photographic field for images which are based on digital image data. A series of different image reproducing devices are used for the reproduction, especially in the photographic field, besides monitors, especially image reproducing devices which will reproduce the image on a medium. Different printers or writing devices are especially used which reproduce images on radiation sensitive media (light sensitive media) by beams (for example, light beams or particle beams or electrode beams). It is common to all image reproduction techniques that image data which define the image to be reproduced, are converted into image data which are suited for the control of the image reproducing device so that the image reproducing device produces the most exact as reproduction possible of the image defined by the original image data.
The present invention relates to devices and processes wherein an image (especially bitmap image) is reproduced with a multitude of individual image points and the image data describe the nominal properties of the individual image points. The properties of the image points are appearance properties, which describe the visual impression which the image point at a certain position in an image has on a human observer, or which, in other words, influence or determine the visual or optical appearance of the image point in the image. The appearance properties of the image points include especially the geometric properties of the image point, such as its relative position (distance) to the adjacent image points or deviation from a nominal position and image point size and image point shape. Properties of the image point are, for example, also its color value or its position in the color space, which can be described, for example, by a lab vector, such as, for example, color, color saturation, color tone, brightness, luminance, and so on. The appearance properties can also be defined for a defined illumination of the image, for example, with a standard light source (for example. D65) at a given aperture angle. Appearance properties can also be determined by the color values of adjacent image points of an image point and their color interaction for the observer as well as by properties of a medium (for example, glossy paper, matte paper) especially with respect to the reflection of an illumination light.
Prior processes for the image reproduction rely on the image reproducing device always answering the same to each arriving image data, irrespective of the location or position of the image point in the image. For this purpose, so called profiles of the image reproducing device are generated to reproduce, for example, a color defined by the arriving image data in a desired manner. Irrespective of the location where the image point is produced, input GB data are converted, for example, into exiting CMYK data for each image point and in the same manner.
This approach produces satisfying results when the image reproducing device answers in the same manner to incoming image data for each image point to be reproduced. However, this is not the case in reality. For example, in an inkjet printer, not all image points are reproduced at the intended position, with the intended dot size and with the intended color value. The actual appearance properties of the image points therefore do not correspond (depending on the position) with the desired appearance properties. The same applies to the other above-mentioned image reproducing devices. For example, the different lasers of a laser printer can produce different image point sizes or the LCD transistors of the monitor can be unevenly spaced.
In order to prevent such errors in the image reproduction as much as possible, a high mechanical effort is made in order to guarantee the most uniform answer possible of the image reproducing device to all the image points to be produced, or one attempts an averaging of the deviations in the control of the image reproducing device. Examples for ink-jet printers are found, for example, in U.S. Pat. No. 5,844,585 and U.S. Pat. No. 5,289,208, the disclosures of which are hereby incorporated by reference in their entireties.
The problems of the prior art are discussed in more detail in the following with reference to inkjet printers. A frequent problem in inkjet printers is the incorrect placement of image points (dots) which is produced by an incorrect orientation or different ejection speeds of the individual nozzles. Furthermore, the individual nozzles can produce different dot sizes. If several printing heads are used, the exact orientation of the printing heads to one another also constitutes a problem. In order to solve these problems, a frequent cleaning of the nozzles was suggested at least in part A solution for an increase in the accuracy of the mutual orientation and control of the printing heads was suggested (U.S. Pat. No. 5,289,208). Further suggested were multiple printing passes with the same nozzles and in the same printing region (U.S. Pat. No. 5,844,585).
However, with the suggested solutions, the printing speeds decreased in the case of several printing passes, the ink consumption increases in the case of frequent cleaning and the cost increases in the case of an increased mechanical precision.
It is now an object of the invention to provide an improved image quality in the image reproduction even at a lower mechanical precision of the image reproducing device.
This object is achieved in accordance with the invention with a process for the reduction of artefacts in reproduced images wherein control values for the reproduction of the image points by the image reproducing device are set based on the image data associated with the image points and taking into consideration the influences on the appearance properties of the image points to be reproduced. The influences are determined based on the device characteristics and depending on the position of the image points.
In the process in accordance with the invention, at least part of the appearance properties (such as, for example, color value and/or position deviation) of the individual image points, which result with a specific image reproducing device, are used as the basis for the reduction of artefacts in the image reproduction. It is a further advantageous aspect of the invention that in particular the position dependent influence (image point position) on the appearance properties is considered by way of the characteristics of the image reproducing device (device characteristics). The appearance properties are determined, especially position dependent, by way of a test image which was produced by the specific image reproducing device. An optical measuring device, such as a camera (for example a digital camera) is used herefor.
The test image data forming the basis of the test image are preferably selected such that at least a large part of the possible different appearance properties of an image point are represented in the test image or the test images. The measurements are preferably carried out in such a way that influences on the appearance properties such as, for example, deviations of the position of an image point from its nominal position, changes of the spacing of an image point from its neighbours, changes of the color density of an image point, changes in the shape of an image point and/or changes of the color value of an image point can be derived from the measured appearance properties.
The measurements for the determination of the appearance properties of the image points can be simplified depending on the type of image reproducing device used. For example, it is sufficient with an inkjet printer to characterize the characteristics of a nozzle in order to predict appearance properties of image points produced with the same nozzle even at other positions of those image points which are produced with the same nozzle.
Preferably the test image points are selected such that at least a large portion of the position dependent changes of the appearance properties of the image points for a specific image reproducing device are captured. Thus, the test image points are preferably selected such that they characterize the position dependent characteristics of the specific image reproducing device during the reproduction of images.
For example, if one captures in the above mentioned manner the appearance properties of image points which are produced by a specific image reproducing device in answer to incoming image data to be reproduced, and if further the dependence of the appearance properties of the image points from the position of the image points in the image is captured, that information which characterizes the specific image reproducing device (herein also called device characteristics) are input into or made available to the process in accordance with the invention. If the process in accordance with the invention is realized, for example, as a program, the characterizing information or the device characteristics can be stored in a memory which is accessed by the program or can even form part of the program code.
The device characteristics influence appearance properties of the image points produced on the basis of reproduction image data. They describe especially the position dependent influence which is especially typical for the image reproducing device. The device characteristics describe especially the influence on the appearance properties. depending on the properties of the image reproducing device. The properties of the image reproducing device are fixed especially by the hardware and/or software of the image reproducing device which processes the image data, for example by the properties of the image dot producers (see further below).
Once the device characteristics which characterize a specific image-reproducing device are available, the following steps are carried out in accordance with the invention. Image data are input which are to be used as a basis for the reproduction of an image and, thus, are referred to in the following as xe2x80x9creproduction image dataxe2x80x9d. The reproduction image data set the nominal position and the nominal properties of the image points to be reproduced for each individual image points in the image to be reproduced. The reproduction image data describe a desired image or nominal image which is to be reproduced as exactly as possible. The reproduction image data processed according to the process in accordance with the invention are referred to as control values which describe especially the actual properties of the image to be reproduced and with which especially an image reproducing device is controlled.
The input reproduction image data are used to determine or calculate the appearance properties of the image points to be expected at the different positions. The device characteristics which characterize the specific image reproducing device are used during the determination to determine or calculate the appearance properties to be expected for (each) image point and for all positions or at least a portion of the positions of the image points. The positions can be determined, for example relative to the image producing device (for example, device body), relative to an image medium (for example, the edge of a paper or an image display) or relative to the image,
An image to be expected with an uncorrected reproduction can be determined in this manner in a first step, which image is then used in a second step as the basis for a correction of the reproduction image data, in order to set or determine the control values for the image reproduction. The control values can also be determined directly in a single step and directly in consideration of the device characteristics. The control values are preferably produced depending on the appearance properties to be expected for the individual image points. This reproduction is carried out in such a way that upon reproduction of the image on the basis of the control values, an image is produced which especially for a human observer (for example, according to CIE standard) optically or visually has smaller deviations from the nominal image than an image which is produced under non-consideration of the device characteristics, i.e. when the reproduction image data are input, for example, directly into the image reproducing device or are subjected to a conventional half tone process. Thus in particular, the process in accordance with the invention can replace or complement a conventional half-tone process.
The artefacts produced by the characterizing properties or device characteristics of a specific image reproducing device are preferably removed or reduced in that the visual interaction of the appearance properties of neighbouring image points during the observation of the image by a human observer it is taken into consideration. Thus, the individual appearance properties of the image points of the subregion co-operate in determining the (overall) optical or visual appearance of the subregion. This is true especially when the individual image points can no longer be resolved by the observer. The visual or optical appearance of a subregion of an image or the optical property of a subregion is influenced, for example, by the geometric distribution of the image points in the subregion and by the color values of the individual image points. A compression of the image points can, for example, cause a decrease of the brightness of the subregion, if the background of the subregion is brighter than the image points. Relative position displacements between the image points can also change the mean color value of the subregion as it appears to an observer. This is the case especially when half tone processes or zither matrix or dither matrix processes are used for the image production. Halftone processes include not only dither matrix processes but also error diffusion processes, whereby a dither matrix processes operate with delimiting non-overlapping positions and error diffusion processes go through the image image point by image point so that the subregions overlap. Both dither matrix processes as well as error diffusion processes can be used for the production of halftones (intermediate color values).
Preferably, expected optical or visual properties of the subregions are determined or calculated based on the appearance properties to be expected of the image points in the sub region. Control values are then produced on the basis thereof in such a way that the optical properties of the sub regions of the reproduced image deviate less from the optical properties of the sub regions of the nominal image than would be the case upon reproduction without consideration of the device characteristics.
The halftone process can be used in combination with the device characteristics to calculate in a first step an actual image to be expected or a color value to be expected of a subregion or an image point. The image data are then corrected in a second step so that the halftone process delivers a result, which is closer to the nominal value. Alternatively, or additionally, the halftone process, especially the error diffusion process based on the device characteristics can be modified such that the modified halftone process (in one step) provides results which are closer to the nominal result than would be the case with the non-modified halftone process.
The device characteristics of the image producing device which are used as the basis for the process preferably describe at least the three-dimensional properties of the image points and their position dependency. Three-dimensional properties of the image points are those properties of the image points which influence the visual or optical appearance of the image points on the basis of the (relative) three-dimensional location of the image point (to the nominal position) and/or the shape and/or size. The three dimensional properties of the image points include especially the relative location (distance, spacing, angular relation) to the adjacent image points and/or the size of the image point. When the image points which are produced by a specific image-reproducing device have different three-dimensional properties at different locations, even though the image points are based on the same image data (for example the same color value to be produced), artefacts are created. These artefacts produced by the three-dimensional properties are compensated in accordance with the invention by color value changes of the image points so that they are no longer apparent to a human observer. The three-dimensional properties of the image points which are to be expected for an uncorrected reproduction can thereby be determined or calculated on the basis of the reproduction image data and the device characteristics which describe the three-dimensional properties of the image points produced therewith and their position dependency. This determination is preferably carried out position dependent and based on the device characteristics.
When the three-dimensional properties of the image points, which are to be expected, are determined at least as far as a position dependency is to be expected, sub region properties to be expected are determined from the expected three-dimensional properties. For example, the proximity of an image point to an adjacent image point in a sub region is determined. A sub region includes more than one image point.
Based on the sub region properties to be expected, the control values are then calculated or determined. The calculation is thereby carried out such that deviations of the three-dimensional properties of the image points in a subregion (for example, deviations from the nominal position) from the nominal properties predetermined by the reproduction image data (for example no deviation from a nominal position) are compensated by changing at least one color value of an image point in the subregion so that a human observer which observes the mean appearance of the subregion perceives a reduced deviation from the nominal appearance of the subregion. For example, when the three-dimensional properties of the subregion cause image points of the subregion to be spaced further apart than they should be and if this causes for a human observer a brightening of the appearance of the subregion, the color value of at least one image point in the subregion is made darker by the correction.
Image reproduction processes exist wherein a specific color value is assigned to subregions of the image to be produced, which color value was produced by the joint appearance of an image point and adjacent points in the subregion. Examples for those processes are dither matrix processes which represent a subgroup of the halftone image processes. Such subregions which have been associated with a common optical property such as, for example, a color value, are herein referred to as cells. While the above mentioned subregions can also overlap or surround one another and, thus, have any shape the cells are directly adjacent, non-overlapping subregions of the image. The subregions which depending on the process are overlapping or non-overlapping are preferably selected such that they cover or include the whole image or at least the major part thereof. The process in accordance with the invention can be carried out subregion by subregion. The process is preferably carried out sequentially for overlapping subregions. For overlapping subregions, one preferably proceeds image point by image point according to the error diffusion process.
When images are produced by way of such subregions, for example, cells (in the following described purely exemplary for xe2x80x9ccellsxe2x80x9d), a cell is often not completely filled with image points, for example, in the dither matrix processes. In those cases, the space between the image points (for example the sheet background in a printer) contributes to the (mean) visual or optical appearance of the cell. The device characteristics or specific properties of an image-producing device can now cause deviations of the individual image points of a cell from their desired appearance properties. Those deviations of the individual image points of a cell cause a change in the overall appearance of the cell. Changes of the three-dimensional properties of the image points (for example deviations of the location from nominal positions) can thereby lead not only to a change of the brightness of the cell in co-operation with the intermediate space (background), but also more generally to a change of the color value of the cell. Preferably the change of the optical overall appearance of the cell is determined or calculated based on the expected apparent properties of the image points of the cell. Based on the thereby expected appearance property of the cell, those properties of the image points of the cell are then changed by correction of the reproduction image data set, which cause a reduction of the deviation of the overall appearance of the cell from a nominal appearance. During correction of the image data it is of course taken into consideration which appearance properties of the image points can be influenced through a correction of the image data and which cannot. For example, if the three-dimensional property (for example the location relative to adjacent image points) cannot be influenced, but only the color value or the size of the image point, only the appearance properties which can be influenced are changed by correction of the image data so that a reduction of the deviation from the nominal image is achieved. As mentioned above, any direct calculation of the control values can be carried out without going through the calculation of expected properties, for example, within the framework of a halftone process, taking into consideration the device characteristics.
During the generation of the control values, which can correspond to corrected reproduction image data, correction processes are preferably used, which can correspond to corrected reproduction image data, which during the correction of the image data associated with a specific image point, takes into consideration the reproduction image data and especially the appearance properties of the adjacent image points. Especially the effect of the correction of one image datum on the correction of adjacent image data is taken into consideration. Error diffusion processes are especially used herefor. Different correction variants can also be simulated and the optimal variant with the smallest deviation from the nominal image of the cell or the subregion can be selected. A xe2x80x9ccorrectionxe2x80x9d means herein that, different from the prior art, device characteristics are used during the determination of the control values.
Whether a correction is carried out or nor can be linked to threshold values or tolerances. If the threshold values or tolerances are exceeded, the correction is carried out. The threshold values or tolerances can be selected for the whole image or partial regions or subregions or cells of the image.
Especially, maintenance processes can be carried out on the image reproducing device, such as, for example, the cleaning of nozzles in inkjet printers, when the tolerances or threshold values are exceeded. For example, only if the execution of the maintenance processes still does not lead to a lowering below the threshold values, the correction is carried out.
Furthermore, warning signals can be output when even after carrying out the correction or maintenance, the tolerances or threshold values are still exceeded which describe the deviation of the nominal image from an image produced on the basis of the corrected image data.
Certain image reproducing devices, such as, for example, inkjet printers or laser printers, use several image point producers (for example, nozzles or lasers), which produce a multitude of image points in an image. The individual image point producers can have different characteristic properties which determine the device characteristics of the image reproducing device. For example, the nozzles of an inkjet printer, or a laser beam of the lasers of a laser printer can be oriented not exactly parallel to one another. This then leads to position dependent appearance properties of the image points or pixels. In order to capture those, test images can be measured with an optical measurement device, which show at least the majority of the image points reproducible with the different image point producers.
The measured image points and their appearance properties are then assigned to the individual image point producers in order to characterize the latter. The appearance properties of the image points expected upon a production based on reproduction image data are determined by finding out by which image point producers the individual image point is produced. Based on the characteristic properties (also called xe2x80x9ccharacteristicsxe2x80x9d) of the image point producer, the appearance properties are then determined for those image points which are to be produced with the image point producer.
In the process in accordance with the invention, the device characteristics can be determined prior to input of the reproduction image data for all positions from the characteristics of the image point producer and their association with positions in an image to be reproduced. However, the assignment of the image point producers to positions can also be carried out only during the determination of the expected appearance properties of image points, which are based on reproduction image data. This should be preferred especially when the position dependency of the device characteristics is dependent on the reproduction image data, for example, when dependent on the reproduction image data it is determined which image point producers are used for which positions. In that case, a determination of the expected appearance properties from the image points to be reproduced is carried out based on the input reproduction image data. The input reproduction image data determine which image point is produced with which image point producer. The expected appearance properties of the image points are then determined from the characteristics of the image point producer used for the production,
The invention further relates to a program which during execution on the computer carries out the process in accordance with the invention. Device characteristics can thereby be provided to the program and/or can be part of the program code.
Image reproducing devices in accordance with the invention preferably include a control device which carries out the process in accordance with the invention. The corrected reproduction image data produced are used for the production of the image by an image reproducing device. The specific image reproducing device which has the device characteristics is preferably the same image reproducing device which is used for the image reproduction or an image reproducing device of similar type and therefore with similar specific properties.
Photographic labs, such as, for example, large labs or minilabs, have at least one image reproducing device in accordance with the invention in order to reproduce photographic image data. Minilabs are small photographic labs with a floor coverage of few square meters and especially a floor coverage of less than one square meter. In a photographic lab, image information is input digitally, for example, through digital data carriers or through a network or also classically (analog), for example, by way of films. The input image information is digitized in order to obtain digital information. The digital image information is then processed, whereby the image reproducing device in accordance with the invention or the process in accordance with the invention is used.
The invention further relates to the use of the process in accordance with the invention, the program in accordance with the invention or the image reproducing device in accordance with the invention, in the field of photography for the production of photographic illustrations or for the output of the corrected, photographic reproduction image data.
The output of the control values (for example, corrected reproduction image data) is carried out preferably through a data interface or a data carrier, for example, CD""s or DVD""s or other digital data carriers. The output can also be achieved by way of the digital interface to a network, especially a LAN or the Internet. The control values can be represented, for example, in any color space (for example, RGB, CMYK, lab, . . . ) and, for example, can present density values.
For example, an end user or a photo business can send test images to a central photo lab in accordance with the invention which allow for the determination of the device characteristics of the image reproducing device used (for example, inkjet printer). A program in accordance with the invention can then be made available based hereon to the end user or the photo business, which program is adapted to the device characteristics of its image reproducing device. If reproduction image data are received from the end user, corrected reproduction image data can also be produced which are adapted to the image reproducing device (printer) of the end user and are sent to the latter, for example, through the Internet. The end user can then use the so produced image data as a basis for the reproduction with its image reproducing device (printer, monitor, etc.) in order to thereby be able to observe an image optimally adapted to the image reproducing device.